The present invention relates generally to a particular class of low molecular weight, relatively monodisperse polyurethane polyols which are suitable for use in high performance, high solids coatings.
Many of the high performance, high solids automotive coatings presently in use are based upon polymeric systems comprised of either polyester-based or polyacrylic-based polyols and crosslinking agents therefor. These coatings are generally supplied as "one-pack" or "two-pack" systems.
In a typical one-pack system, all of the coating ingredients are combined into one storage stable mixture. Upon application the polyol component is crosslinked, generally with an aminoplast resin (such as a melamine resin) or a blocked isocyanate, under heat cure conditions of 120.degree. C. or above. In a typical two-pack system, the polyol component is combined with a crosslinking agent, generally an isocyanate, shortly before application, with curing being conducted at ambient or elevated temperatures.
For environmental reasons, it is becoming increasingly important to develop polymeric systems with low solution viscosities, which permit the formulation of high solids coatings with low application viscosities suitable for spraying. High solids coatings (generally about 50 wt % or greater solids) significantly decrease the amount of volatile organic compounds (VOC) entering the atmosphere upon drying/curing of the coating.
To achieve acceptable solution viscosities (20-30 seconds. #4 Ford Cup at 20.degree. C.) for typical high solids coating systems, the polyols should possess a weight average molecular weight (Mw) of about 5000 or lower. In general, the lower the Mw the lower the solution viscosity.
To achieve good film properties it is important that, upon film formation, the polyol molecules become sufficiently chemically bonded to each other. This can be accomplished by providing each polyol molecule with at least two reactive hydroxyl groups. A very low hydroxyl equivalent weight (HEW) (e.g., below about 200), however, may lead to brittle films. It has been found that, in general, the best spectrum of film properties may be obtained for HEWs between about 300 to 500. It follows, therefore, that for good film formation the polyols should possess a number average molecular weight (Mn) of at least about 800.
As is evident from the above discussion, the requirements for acceptable solution viscosities and good film properties lead to contradictory molecular weight requirements--for low solution viscosities the Mw should be low, but for good film properties the Mn should be high.
In acrylic free radical polymerization and in polycondensation leading to polyesters, it is difficult to achieve sufficiently narrow molecular weight distributions. In other words, it is difficult to formulate high solids, high performance coating systems from acrylic and/or polyester based polyols which possess both acceptable application viscosities and resulting film properties.
A considerable amount of work in this area has recently been done relating to high solids, high performance coatings which are based, in part, upon relatively low molecular weight polyesterurethane, urethane-modified polyester and polyurethane polyols.
For example, U.S. Pat. Nos. 4,485,228 and 4,540,766 describe high solids coating systems based, in part, upon relatively low molecular weight polyesterurethane polyols and crosslinking agents therefor. More particularly, U.S. Pat. No. 4,485,228 describes a two-pack system with a polyisocyanate crosslinker, while U.S. Pat. No. 4,540,766 describes a one-pack system with an aminoplast or blocked isocyanate crosslinker. The polyesterurethane polyols of these references are produced via the reaction of a polyisocyanate with a stoichiometric excess of a polyester polyol.
In related U.S. Pat. Nos. 4,540,771 and 4,605,724, the polyester polyols utilized for the polyurethane polyols are produced by reacting a polycarboxylic acid or lactone with a large excess of a low molecular weight polyol wherein, after completion of the reaction, the excess polyol is removed, e.g., by distillation.
U.S. Pat. No. 4,548,998, like those references just mentioned, describes a high solids coating system based upon a polyesterurethane polyol, except that the polyesterurethane polyol is produced by isocyanate-free reaction of a polyester polyol, urea and a polyamine.
U.S. Pat. Nos. 4,524,192, 4,530,976, 4,533,703, 4,533,704 and EP-A-0139513 describe similar high solids coating systems which are based, in part, upon urethane-modified polyester polyols and crosslinking agents therefor. The urethane-modified polyester polyols are produced by reacting a urethane-modified diol component (from a diol and diisocyanate) with a diacid component and a second polyol including at least 5 wt % triol.
In all of the aforementioned systems, molecular weight control of the polyesterurethane and urethane-modified polyester polyols is difficult because unwanted chain extension beyond the intended low molecular weight often results, leading to undesirably broad molecular weight distributions and high degrees of dispersion. The consequence, as indicated above for the acrylic and polyester based coatings, is that it is difficult to formulate high solids, high performance coating systems which possess both acceptable application viscosities and resulting film properties.
The problem of molecular weight control is somewhat alleviated by the process taught in U.S. Pat. No. 4,543,405. More particularly, this reference describes high performance, high solids coatings based upon low molecular weight polyurethane polyols and/or higher molecular weight prepolymers thereof, which polyurethane polyols are produced by reacting a polyisocyanate with a large excess of a polyol. After completion of the reaction, the excess polyol is removed, e.g., by distillation. Also relevant in this aspect is U.S. Pat. No. 4,288,577.
An obvious disadvantage of this procedure (and that of aforementioned U.S. Pat. Nos. 4,540,771 and 4,605,724) is that the distillation off of the excess polyol is inconvenient. Further, even at a high stoichiometric excess of the polyol reactant, unwanted chain extension beyond the intended low molecular weight may still occur.
It would, therefore, be advantageous to provide a polyol suitable for use in high performance, high solids coatings which not only possesses a desirable spectrum of molecular weight properties but also is quite convenient to prepare.